1. Field of the Invention
The present invention relates to a high efficiency gas turbine system and a combined plant comprising this gas turbine system.
2. Description of the Prior Art
One example of a gas turbine system and a combined plant comprising this gas turbine system in the prior art will be described with reference to FIG. 3. FIG. 3 shows a main construction of a prior art example of a combined plant comprising gas turbine system and a steam turbine.
In FIG. 3, combustion air a is compressed at a compressor 01 and is then mixed with fuel b at a combuster 02 to effect a combustion for generating a high temperature combustion gas. This high temperature combustion gas drives a gas turbine 03 to expand and is then supplied into an exhaust gas boiler 05 as exhaust gas c for generating steam there.
A high temperature high pressure steam generated at the exhaust gas boiler 05 drives a steam turbine 06 to expand and is then condensed at a condenser 08. The pressure of the condensed water is elevated by a feed water pump 09 so that the condensed water is circulated into the exhaust gas boiler 05.
A generator 04 is fitted to the gas turbine 03 and another generator 07 is fitted to the steam turbine 06 and so the construction is made such that work each of the turbines 03, 06 may be taken out.
In the combined plant as so constructed, there is currently a high demand for realizing a higher efficiency thereof. Especially, in order for a gas turbine system which constitutes a first stage portion of the combined plant to effect a high temperature combustion of the fuel, further enhancement of the efficiency and reduction of NOx generation are both required. Also, if the gas turbine system comprises a gas turbine unit only, enhancement of the efficiency and reduction of NOx generation are likewise required.
In view of the mentioned high demand for realizing a higher efficiency and a further reduction of NOx generation in the gas turbine system and the combined plant comprising the same in the prior art, it is an object of the present invention to provide a more effective gas turbine system and combined plant comprising the same.
In order to achieve the mentioned object, the present invention provides the following embodiment.
In a first embodiment, a gas turbine system comprises a compressor for compressing combustion air, a combustor for burning fuel with the combustion air and a gas turbine driven by a high temperature gas generated at the combustor. The gas turbine system is constructed such that a portion of exhaust gas discharged from the gas turbine is recirculated into the combustor.
In the conventional gas turbine system, the combustion gas has a high excess air ratio due to a limitation in the turbine inlet temperature and because the exhaust gas has a high oxygen concentration. However, in the first embodiment of the present invention, a portion of the exhaust gas is recirculated into the combustor and thereby the oxygen concentration in the exhaust gas can be reduced, exhaust loss of the gas turbine system is largely reduced, and generation of NOx is also reduced. Further, as the exhaust gas to be circulated into the combustor has a high steam concentration, work output of the gas turbine is increased because of the physical properties of the high steam concentration.
In a modified embodiment, a gas turbine system as mentioned above further comprises an exhaust gas compressor and is constructed such that the portion of exhaust gas is pressurized by the exhaust gas compressor to be partly recirculated into the combustor and to be partly mixed into the fuel before the fuel is supplied into the combustor.
In the conventional gas turbine system, the combustion gas has a high excess air ratio due to a limitation in the turbine inlet temperature and the exhaust gas has a high oxygen concentration. However, in the present invention, the exhaust gas is circulated into the combustor and thereby the excess air ratio of the combustion gas can be made lower. Hence, oxygen concentration in the exhaust gas can be reduced and exhaust loss from the gas turbine system can be reduced largely.
Also, the exhaust gas of the low oxygen concentration and the fuel are mixed together before the fuel is supplied into the combustor so that a low BUT gas may be formed, and this low BUT gas is mixed with air to burn. Hence, flame temperature can be maintained lower and NOx generation quantity can be reduced. Moreover, the exhaust gas recirculated into the combustor or the fuel has a high steam concentration, work output of the gas turbine is increased because of physical property of the high steam concentration.
In another modified embodiment a gas turbine system as mentioned above is designed so that the exhaust gas pressurized by the exhaust gas compressor is used as cooling gas of the gas turbine.
According to the embodiment above, in addition to the features of the first modified embodiment, the exhaust gas pressurized by the exhaust gas compressor is used as a cooling medium for the gas turbine. Because the exhaust gas has a high steam and CO2 gas concentration, a high cooling effect can be obtained because of physical properties of the high steam and CO2 gas concentration.
In another modified embodiment, a gas turbine system as mentioned above further comprises a condenser through which the exhaust gas passes so that water content in the exhaust gas may be condensed, and the portion of exhaust gas is the water content in the exhaust gas so condensed at the condenser.
According to the embodiment above, if the combustion gas in the combustor has an excessively high temperature even if the excess air ratio is set to a lower level, water injection or steam injection is effected using the water content in the exhaust gas to be circulated into the combustor so that a predetermined turbine inlet temperature may be obtained and the excess air ratio of the combustion gas may be reduced. Therefore, oxygen concentration in the exhaust gas can be reduced and exhaust loss can be largely reduced as compared with the conventional gas turbine system.
Also, flame temperature and oxygen concentration in the combustor can be maintained lower so that NOx generation quantity can be reduced. Moreover, water content concentration in the combustion gas becomes higher so that work output of the gas turbine is increased because of the physical properties of the higher water content concentration.
It is to be noted that if the oxygen concentration in the exhaust gas is maintained lower, then necessarily steam concentration in the exhaust gas becomes higher. Hence, the water content in the exhaust gas can be recovered easily at the condenser so that the water injection or steam injection becomes possible without a make-up water.
Another embodiment is directed to a combined plant comprising the gas turbine system as mentioned in any one of the above embodiments.
According to the embodiment above, a combined plant having a feature of the gas turbine system mentioned in any one of the previous embodiments can be obtained, and a combined plant having a high efficiency and a low NOx generation as a whole can be realized.